DESCRIPTION: In typically-developing children initially slow, one-at-a-time word learning is followed by an apparent explosion in their lexicon. One estimate suggests that children are learning up to nine new words a day. By contrast, for some individuals with severe mental retardation new words enter the lexicon only through slow and deliberate effort. Yet word learning is a foundational component of human development, necessary for advanced linguistic functions and contributing to complex representational skills (e.g., categorization). To the extent that a cognitive disability interferes with word learning, progress in these domains will be correspondingly limited. It is therefore essential to explore in detail the nature of lexical impairments in severe mental retardation. The existence of difficulties in lexical acquisition among individuals with severe mental retardation raises important questions. When rapid vocabulary expansion occurs, what linguistic cognitive processes support that learning? When it does not occur, what skills are deficient or absent? A phenomenon called fast mapping may be of greatest potential relevance for rapid vocabulary expansion. Fast mapping refers to a quick, initial partial understanding of a new word's meaning derived from the context of word use. Fast mapping has been argued to facilitate the vocabulary explosion. This proposed role has received empirical confirmation in typical youngsters and children with Down syndrome. Yet evidence from children with atypical cognition/language has recently challenged the universality of this relation. The Principal Investigator's studies have added to the preliminary evidence of selective deficits in fast mapping among individuals with severe mental retardation, William's syndrome, or specific language impairments. The deficits are most often observed when individuals attempt to apply the principle for the purpose of vocabulary expansion. These recent findings oblige a more systematic analysis of the precise role of the principle in learning outcomes in mental retardation. Is fast mapping at risk in individuals with severe mental retardation, potentially limiting their vocabulary expansion? This application proposes a five-year study of vocabulary expansion and delay, focusing specifically on rapid expansion of an extant, but limited, vocabulary. The proposed series of studies will implement methods developed by the Principal Investigator that will enable the systematic analysis of unresolved questions of lexical expansion in severe mental retardation. The project has four specific aims that will be addressed in the course of four studies: 1) To examine the relation of fast mapping, rapid vocabulary expansion, and nonverbal processes, in order to explore the nature of lexical expansion in severe mental retardation; 2) To explore the points of greatest vulnerability for learning through fast mapping by explicitly taxing participants' skills and observing the ways in which breakdowns occur; 3) To extend the analysis from the commonly examined object-word learning to the acquisition of action-related words, to determine whether processes of fast mapping are similar for the two types of form/class categories; 4) To examine the basis underlying children's apparent assumption that new words should and do map to unnamed objects, and whether this assumption is intact among individuals with severe mental retardation.